Chemical A0-3959X.91 – 15, also known as Agent A0-3959X.91 – 15[1] and referred to colloquially as the "black goo" or "black liquid", is an extremely potent and virulent mutagenic pathogen, composed of millions of small micro-organisms, that was manufactured by the Engineers as a biological weapon, presumably for military purposes. Due to its volatile nature at ambient temperature, the Engineers housed the pathogen within Steatite Ampules.

The primary purpose of the Chemical A0-3959X.91 – 15 pathogen is to cleanse planets of unwanted non-botanical life forms. Once exposed to another organism, the virus begins mutating its host by rewriting their DNA. The host becomes exceedingly aggressive and seemingly mindless as it attacks any living thing in sight. Geologist Fifield was exposed to the pathogen during the USCSS Prometheus' expedition of LV-223, mutating his appearance and causing him to attack his fellow crew members. He was also extremely resilient to physical injury, being able to withstand multiple gunshots. It was not until he absorbed multiple shotgun blasts, was scorched repeatedly with a flamethrowerand ran over twice with an RT Series Group Transport that he was finally killed. The pathogen also mutated several worms found in the Engineer temple, turning them into the aggressive Hammerpedes. These creatures exhibited acidic blood and the ability to regenerate lost body parts, though the latter is an ability commonly found in flatworms on Earth. However, the regeneration of Hammerpedes is much faster than that of normal flatworms, suggesting that the pathogen can enhance a host's natural abilities.

Charlie Holloway was also exposed to the pathogen and began mutating before Meredith Vickers killed him to prevent the virus from coming aboard the Prometheus. The virus appeared to affect his reproductive system. Prior to his physical mutation (while carrying the virus), Holloway was engaged in sexual intercourse with his lover, Elizabeth Shaw, impregnating her with the Trilobite. It is not clear if his sperm was mutated causing an abnormal conception or if he was simply carrying a Trilobite that moved into Shaw, though the latter seems more probable given that Shaw was sterile. Whether Trilobites prefer to move into female subjects or if Holloway would have naturally given "birth" to the Trilobite is also unknown. Although Shaw was exposed to the infected Holloway and had sex with him, she was not infected with the liquid and only received the Trilobite embryo. This suggests that the Black Liquid itself is not sexually transmitted, but it affects the host's reproductive system.

The chemical's induced mutations seems to be random with it never creating the same effect, however, it often does seem to accelerate the healing ability of its host though anyone or anything with cancer will have their condition greatly accelerated to the point of eventual death.

According to David, who spent a decade extensively studying the pathogen and its effects, the virus is designed to infect non-botanical organic life. As demonstrated on Planet 4, the virus is also capable of killing outright instead of merely infecting. When the pathogen infects a viable host it gestates a hybridized parasitic life form which may kill the host upon emergence. These hybridized creatures — examples including the Trilobite on LV-223 (from Holloway, which passed to Shaw), the insect-like motes on Planet 4 from the mutated fungus, and the parasites which burst from the dying Engineers on Planet 4 after David released the virus. These parasites then seek out another host to attack or infect if possible. Once infected, the host will gestate and birth some form of hybridized creature bearing Xenomorph-like qualities. The appearance of these hybridized creatures depends on the host organism due to a DNA Reflex process, though they possess common traits such as elongated eyeless heads and a tendency for acidic blood. Examples include The Deacon on LV-223 and the Neomorphs on Planet 4. In the absence of viable hosts, the virus and the creatures spawned from it can enter an indefinite period of dormancy, becoming undetectable until awakened by a suitable host. David utilized Chemical A0-3959X.91 – 15 as part of his experiments to create the Xenomorphs seen on Planet 4.

The speed and intensity of the mutation process depends on the amount of virus taken on by the host and the manner in which it is exposed i.e ingestation or skin absorption. Charlie Holloway's mutation happened at a slower rate than Sean Fifield's, which was more instantly obvious and horrific. Holloway had only a tiny drop of the black liquid in his drink, but Fifield took a full dose of the liquid in his face when he fell down in the temple. At very high doses, the mutation happens so fast that the body breaks down and it kills the victim quickly e.g. the Sacrifice Engineer, who drank a full cup of liquid though it is uncertain if this is indeed the same type of liquid as the rest.

History

It was shown that the Engineers created the chemical thousands of years ago on LV-223 for the purpose of wiping out entire planets. Somehow, the Engineers themselves were exposed to the pathogen, wiping out the entire Earth invasion force, except for one still in stasis.

In 2093, the USCSS Prometheus came to explore LV-223 after finding clues of the Engineers' existence in ancient cave drawings on the Isle of Skye and in the archeological sites of many other unrelated ancient Earth civilizations. A few crew members were exposed to the pathogen, and in the ensuing chaos most were killed. The remaining crew later sacrificed themselves and the Prometheus to destroy the Engineer's ship, with Elizabeth Shaw and the androidDavid as the only survivors leaving aboard another abandoned ship to head for the Engineer's homeworld.

The lone surviving Engineer on the planet was impregnated by a Trilobite, resulting in his death and the Deacon bursting from his chest.

In researching the aftermath of Project Prometheus, Weyland Industries managed to uncover an information manual written by the Engineers concerning the chemical. Linguistic components from the original document were translated by a Weyland Industries' cybernetic individual. The document was later declassified.[1]

Upon arriving on the Engineer homeworld, David deployed the steatite ampules over a large settlement, the pathogen petrifying every Engineer present and drastically affecting the environment by exterminating or mutating non-botanical organisms, giving rise to highly aggressive hybridized predators like the Neomorphs.[2]

Trivia

At the start of Prometheus, an Engineer is shown to drink a similar-looking liquid which causes his body to disintegrate into DNA. It is unknown whether or not this liquid was the black liquid, though the effects of the two are remarkably different.

The black liquid in the earlier drafts of Prometheus which was written by Jon Spaihts, was originally a swarm of black scarab like insects that dissolved the sacrificial Engineer, and infused the Xenomorph DNA into Fifield which turned him into a form of a xenomorph.This concept was eventually changed when Damon Lindelof was brought in for rewrites.